A distributed temperature sensor (DTS) measures temperatures along an optical fibre that is located in thermal contact with an object to be measured. The principle of operation of a DTS is analogous to that of RADAR and SONAR. In RADAR, the total time that an electromagnetic probing pulse takes to travel from the source to a distant object and that its reflection takes to return to the origin, together with the known speed of the electromagnetic wave, allows the location of a distant object to be deduced. In SONAR an acoustic probing signal is employed. In DTS systems a very short pulse of laser light (less than 100 ns) is used as the probe. After leaving the laser the light pulse travels through an optical fibre to an optical coupler and into the sensing fibre. As the light pulse travels along the sensing fibre, the pulse intensity is attenuated by scattering and absorption in the fibre material.
Modern high-purity optical fibres have a window of low absorption for wavelengths from approximately 0.8 to 1.7 μm. Within this window, the optical losses are dominated by Rayleigh scattering, which contributes the following term to the total attenuation:
            α      R        =                            8          ⁢                                          ⁢                      π            3                                    3          ⁢                                          ⁢                      λ            4                              ⁢                        (                                    n              2                        -            1                    )                2            ⁢              β        T            ⁢              kT        f              ,where λ is wavelength, n is the refractive index, βT is the isothermal compressibility at Tf, k is Boltzmann's constant and Tf is a constant related to the glass anneal temperature. The attenuation coefficient is usually expressed in units of dB/km. The probe pulse is also attenuated by interactions with the optical medium that change the wavelength of the light. Brillouin and Raman scattering are the most important of these relatively weak scattering mechanisms. These interactions cause light to be scattered back towards the proximal end of the fibre as light of different wavelengths. The backscattered power received for Raman scattering at a particular wavelength λs>λ (known as the Stokes component) is given by
            P      s        ∝          1                        λ          s          4                ⁡                  [                      1            -                          exp              ⁡                              (                                                      -                    hv                                    /                  kT                                )                                              ]                      ,while for λas<λ (the anti-Stokes component) the backscattered power is given by
            P      as        ∝          1                        λ          as          4                ⁡                  [                                    exp              ⁡                              (                                  hv                  /                  kT                                )                                      -            1                    ]                      ,where h is Planck's constant, ν is the frequency shift of the scattered light and T is the temperature. Thus, while most light energy is transmitted in the forward direction along the fibre, a small fraction of it is scattered backwards, where it may be detected and analysed. Of the backscattered light, the Rayleigh scattering occurs at the probe wavelength and is relatively insensitive to temperature changes in the fibre, while the Raman scattering is shifted in wavelength from the probe light and has an explicit temperature dependence.
In general, the accurate derivation of quantities based on the measurement of light intensity is most conveniently made through the measurement of light intensity ratios, because the absolute intensity (or power) is difficult to measure accurately. The intensity of an optical signal can be influenced by a large number of variables in addition to the quantity of interest. For example, the power output of the source, the efficiency of the detector and the efficiency of the optical elements could all be affected by changes in ambient temperature or humidity. Some components may be subject to ageing effects. These effects can be difficult to predict or identify and are therefore difficult to model reliably. Alternatively, the effects can be reduced through the provision of a stable thermal environment and suitable calibration means. In the art, it has been argued that various combinations of these approaches (ratiometric and calibration) can provide practical and efficient solutions to obtaining accurate measurements of temperature distribution, given all of the various sources of uncertainty that apply.
The method of detection and analysis varies between different DTS embodiments based on glass optical fibres. In the earliest embodiments, a diffraction grating was used to filter out a band of backscattered wavelengths close to the laser wavelength (mainly the Rayleigh scattering). The Stokes and anti-Stokes Raman wavelengths were allowed to pass to separate detectors and the intensity ratio of these components was used to derive the temperature as a function of range in the fibre (see GB 2,140,554A).
An improved method was subsequently devised, whereby the Rayleigh scattering and anti-Stokes Raman scattering are selected for measurement by separate detectors (see GB 2,183,821A). These intensities are compared in a ratio device to give an indication of the temperatures in the fibre. It is claimed that this arrangement permits a much faster response than the prior art method, as the Rayleigh scattered light is much more intense than the Stokes Raman and can be sensed using relatively simple and inexpensive equipment.
In a further development, a method was devised whereby a single spectral band of the backscattered radiation (usually a region of the broad anti-Stokes spectrum) is selected for analysis (see U.S. Pat. No. 4,823,166). The method uses a calibration function to deduce the temperature distribution from the measured backscatter power. The data conversion may be carried out either using a tabulated variation of the backscatter factor with temperature, or via a theoretical model that relates absolute temperature to intensity as a function of λas.
It is claimed that the embodiments described in U.S. Pat. No. 4,823,166 remove the need for corrections to be made for the difference in fibre attenuation between the Stokes and anti-Stokes wavelengths. It is also claimed that the system offers enhanced sensitivity to temperature changes, reduced sensitivity to drifts in the source wavelength and a simplified optical arrangement. Short-term changes in the energy and wavelength of the source can be detected and corrected by monitoring a short reference section of the fibre that is held at a constant temperature in a temperature-controlled chamber. However, since this approach relies on the accurate measurement of intensity in a single spectral band, its effectiveness is critically dependent on the elimination of variations in backscatter factor that arise from non-temperature (NT) factors. In particular, axial variations in the fibre loss are of particular concern in the current context. These are typically associated with built-in or acquired defects in the fibre that cause temperature-independent variations in the scattering coefficient.
Specification U.S. Pat. No. 4,823,166 suggests three ways in which the effects of variations in the fibre loss may be eliminated from the measured temperature distribution. The first method involves performing the measurement from each end of the optical fibre. The effects of any propagation losses are eliminated by calculating the geometric mean of the backscatter signals measured from both ends of the fibre and returning from a particular location. Unfortunately this approach adds to the instrumental complexity and is less convenient to deploy than a single-ended measurement arrangement.
A second approach involves calibrating the entire fibre before installation with a known temperature distribution. The sensor then measures departures of the backscatter intensity from those determined at the time of calibration and interprets them in terms of a temperature variation. However, this approach restricts the system to use with fibres for which a calibration has been performed and requires recalibration if the fibre properties change. The third approach makes provision for the removal of the filter to facilitate measurement of the total backscatter signal in the reference section, or over the entire fibre length, so that a normalisation can be performed. The total backscatter signal is dominated by the Rayleigh scattering, which is relatively temperature insensitive, but sensitively reflects the fibre loss characteristics. However, the need to remove the filter to perform the normalisation procedure adds to the complexity of the optical system and remains a drawback for practical operation.
It is these issues that have brought about the present invention.